The present invention is directed to a bone fixation assembly, and in particular to a bushing and screw assembly for securing a fixation device to bone.
Orthopaedic fixation devices, both internal and external, are frequently coupled to bone by the use of fasteners such as screws or pins. For example, bone plates can be secured to bone with bone screws inserted through plate holes. Securing the screws to the plate provides a fixed angle relationship between the plate and screw and reduces the incidence of loosening. One method of securing the screw to the plate involves the use of so-called xe2x80x9cexpansion-head screws.xe2x80x9d U.S. Pat. No. 4,484,570 discloses an expansion-head screw with a head that has a recess, the walls of which contain a number of slits. After the expansion-head screw is inserted into bone through a hole in the fixation device, a locking screw is inserted into the recess to expand the walls of the recess to thereby lock the screw to the fixation device.
In addition to securing the screw to the fixation device, it is also often desirable to insert the screws at an angle relative to the fixation device selected by the surgeon. The prior art discloses a number of these so-called xe2x80x9cpolyaxialxe2x80x9d systems, most of which utilize a bushing located in a hole in the fixation device to provide for angulation of the screw relative to the fixation device. For example, U.S. Pat. No. 5,954,722 discloses a polyaxial locking plate that includes a plate hole having a bushing rotatable within the hole. As a screw is being inserted into bone through the bushing and plate hole, a threaded head of the screw engages a threaded internal surface of the bushing to expand the bushing against the wall of the plate hole, thereby locking the screw at the desired angular orientation with respect to the plate.
The present invention relates to an improved bone fixation assembly that allows for a surgeon-selected angle of a fastening screw relative to the fixation device and locking of the fastening screw once the screw is at the desired orientation.
The present invention relates to a bone fixation assembly for securing a fixation device, such as a bone plate, to bone. The assembly includes a fixation device, a bushing, a fastening screw, and a locking screw. The bushing has a sidewall with an exterior surface configured and dimensioned for polyaxial rotation within the through hole of the fixation device and an interior surface which defines a bore that extends through the upper and lower surfaces of the bushing. The sidewall has at least one slot for allowing outward expansion of the sidewall against the through hole of the fixation device to thereby lock the bushing at a selected angle relative to the axis of the through hole of the fixation device.
The fastening screw has a threaded shaft configured and dimensioned for insertion through the bore of the bushing and threadably insertable into bone to secure the bushing and fixation device to bone. The head of the fastening screw is configured and dimensioned for insertion in the bushing and includes a radial wall and open end defining a recess. The radial side wall has at least one slit for allowing outward expansion of the radial wall thereby outwardly expanding the sidewall of the bushing. The locking screw has a threaded body threadably received in the head of the fastening screw to thereby outwardly expand the radial wall of the fastening screw.
The bushing can have a substantially frustospherical shape with the bore extending through the central axis of the bushing, perpendicular to the upper and lower surfaces. In an exemplary embodiment, the bushing has an annular shoulder and the head of the fastening screw is flush with the upper surface of the bushing when the head of the fastening screw rests against the shoulder. The fastening screw can be provided with a lip protruding from the radial wall which rests on the annular shoulder when the fastening screw is threaded into the bone.
In one embodiment, the sidewall of the bushing includes a plurality of slots extending from the upper surface of the bushing. Furthermore, one of the slots can extend from the upper surface completely through the lower surface. In order to further increase the security of the bushing within the through hole, the exterior surface of the sidewall of the bushing and/or the walls of the through hole of the fixation device can be provided with ridges.
In order to eliminate the need to tap a drilled hole, the fastening screw can be a self-tapping screw. The fastening screw could also be a self-drilling screw. In an exemplary embodiment, the head of the fastening screw includes a tapered portion proximal to the threaded shaft.
In one embodiment, the recess of the fastening screw has a bottom surface with a threaded hole for threadably receiving at least a portion of the threaded body of the locking screw. The locking screw can have a tapered end. Additionally, the radial wall of the head of the fastening screw can have a matching tapered inner surface.
The present invention also relates to a method for securing a fixation device having a through hole to bone. A fastening screw is inserted through a bushing located in the through hole of a fixation device. The fastening screw is then threaded into a bone. After the threading step is completed, the fastening screw is locked to the bushing and fixation device. The fastening screw can be inserted through the bushing at a surgeon-selected orientation relative to the fixation device. The fastening screw head is preferably expanded against the bushing to thereby lock the bushing to the fixation device. A locking screw can be inserted into a recess in the fastening screw head to cause the expansion.